I tend to see more half bottles in the Cambridge or Somerville area. Cambridge Wine and Spirits often has a couple. Wine and Cheese Cask might have some as they have in the past, but I haven't been in ages. I found one of Ideal's Spanish lines in a half bottle at the liquor store across from Somerville Hospital once, but I really doubt its there anymore. Gordons and Martignetti's carry their French and Italian wines, but I can't recall seeing any Spanish half bottles.
If you are willing to make arrangements to order a case, there are a number of Portuguese half bottles which aren't common in stores, but some are distributed through larger distributors. Definitely Monte Velho and Alandra which are mid-lines (regional wines, not DOC, not table wines) from Herdade do Esporao. They are both from the Alentejo, more new world styled wines. American Oak and Tempranillo in the blend for the Monte Velho which is more fruit forward, the Alandra I believe is unoaked and more basic portuguese grapes. Touch sweeter than most wines I drink. I would think that Rapid could get these and Kappy's probably has the 750 ml of the Monte Velho to try. Marques de Borba another Alentejo wine with tempranillo is available as a half-bottle I think is distributed by Signature imports, which Kappys could probably get. A bit harder to get another Alentejo producer Adega Cooperativa de borba has their two midline wines in 375ml bottles both of which are quite traditional portuguese blends with some tempranillo. New England Meat Market in Peabody probably carries their normal bottles. Quinta de Aveleda has vinho verde, plus a red or two, including the only Douro I have seen in a half bottle Charamba. Monte Velho is probably your best bet for as easier to get and drinkable, Charamba might be worth a try if you can find it. I would try them as a 750 ml, the Adega C de Borba wine is closer to a table wine but more of a traditional portugese style. Meia Encostra is another regional from the Dao which is available on the south coast in a half bottle. I don't know any single "cast" Portuguese wines, but until you setup up from the mid-range wines, all are blends of 3-4 grapes and all regions have a wide variety of Portuguese grapes grown. In any case you should be able to check if any of these work for $7-$11/750mL or less, anything more than that someone is over charging you. Between 660, Martin Brothers, and Prospect Liquors all on Cambridge street you could probably try everything on this list, plus Portugalia offers a few half bottles (more vinho verde) that you could check out while having something simple at their bar. Alexander liquors could get some of those to their Medford store with some advance notice (Somerville has several, but Medford has less) -- they deliver at least as far as Malden. The half bottles are really common in New Bedford/Fall River where they are sold to lunch type restaurants, where you can order them "cold" (bud lite cold, how many Portuguese order red wine) or warm, so you will see tables of a dozen or so workers each with a different half-bottle.
Wine and Cheese Cask
407 Washington St, Somerville, MA